The present invention is directed towards an improved combustion liner construction and more specifically to a combustion liner construction including a structural outer shell and a reduced diameter throat insert flexibly coupled thereto.
An improved dual stage-dual mode low NOx combustor is described in co-pending application Ser. No. 56,510 filed July 11, 1979, in the name of Colin Wilkes and Milton B. Hilt, now U.S. Pat. No. 4,292,801 and of common assignee as the present invention. This application discloses a liner assembly including a reduced diameter throat region which divides a gas turbine combustor into first and second combustion chambers. As a result of this division into two chambers, it is possible to control the operation of the combustor to insure that under selected conditions burning takes place in the second chamber only. Such a restriction has been found to reduce the exhaust emissions of the combustor and at the same time prevent flashback from the second chamber into the first chamber. The manner in which these advantages are achieved is described in some detail in the Wilkes application whose disclosure is incorporated herein.
While the combustor of the Wilkes application successfully achieves the foregoing advantages, it has been found that the reduced neck section of the combustor liner are subject to adverse thermal and mechanical stresses which is best overcome by the use of a throat insert in accordance with the present invention. The primary object of the present invention is to provide a new double-wall liner structure which maintains the advantages of the Wilkes application while reducing both the mechanical and thermal stresses placed on the throat section and at the same time simplifying the manner in which the throat section may be replaced.
In accordance with the present invention, the double-wall liner assembly includes an outer structural shell and an inner throat insert flexibly coupled thereto. The throat insert divides the combustor into upstream and downstream combustion chambers for the purposes set forth in the Wilkes patent application. A plurality of fingers are preferably formed about the periphery of the upstream end of the throat insert and are each coupled to the outer shell by a plug weld. The upstream fingers are formed in the throat insert by providing a plurality of axially extending slots about the outer periphery of the upstream portion of the throat insert. In addition to defining the upstream fingers, the slots provide a controlled flow of cooling air from a toroidal space defined between the outer shell and the throat region of the insert into the upstream chamber of the liner. These fingers also provide controlled mount flexibility which diverts liner bending loads from the throat directly into the liner shell so as to lessen throat stresses.
A plurality of axially extending upstream fingers are coupled to the downstream end of the throat insert which provides a slidable, flexible interference-fit coupling between the insert and the outer shell, resulting in a radially loaded coupling. The downstream fingers are preferably, but not necessarily, formed from a single piece of sheet metal which is bent in a circle to form an air seal. The air seal fits around the downstream outer periphery of the throat insert and is preferably coupled thereto by a plurality of spot welds. The flexible fingers are biased against the outer shell of the liner and provide several structural benefits. Initially, the preloading of the fingers insures that they will be biased against the outer shell thereby avoiding joint gaps that otherwise would permit gross, uncontrolled air leakage from the toridal space to the upstream chamber of the liner. The preloading of the fingers further induces Coulomb damping as a means of attenuating throat mechanical vibration induced by normal combustion. Finally, since the fingers form a highly flexible joint between the downstream end of the throat insert and the outer casing, this joint is capable of radical deflection and/or axial motion that is impeded only by the friction between the fingers and the outer shell induced by preloading. As a result, there is a substantial reduction in stresses which are produced by differential thermal expansion between the throat insert and the outer casing. In addition, the sliding connection between the throat insert and the outer casing at the downstream end of the throat insert simplifies the replacement of the insert. Particularly, the insert can be slid out of the outer casing once the plug welds formed at the upstream end of the insert are removed.